Tolerance of irradiated animals to prolonged hypoxia

Rats were maintained at a simulated altitude of 15,000 feet for 30 days following exposure to x-ray doses in the 30-day ld10–90 range. Nonirradiated rats maintained concurrently tolerated this level of altitude without apparent distress. The effect of prolonged altitude exposure on the irradiated animals was equivalent to a 100 r decrease in the x-ray dose necessary to produce a 50% mortality response at 30 days. Mice as well as rats exhibited a greater mortality response to a given x-ray dose if maintained at altitude during the postirradiation period. Red blood cell volume, hemoglobin and hematocrit values were determined at intervals during 30 days of exposure to 15,000 feet simulated altitude following 450 r or 0 r, and were compared to those for similar groups maintained at sea level. The blood picture of irradiated animals during the 1st 20 days at altitude more closely resembled irradiated animals maintained at sea level than the altitude controls. These data suggest that the increased mortality of animals maintained at altitude following radiation exposure is the result of a reduced blood cell volume and an inadequate erythropoietic response during exposure to hypoxia.